This invention relates to the field of packaging for respiring items such as fresh fruits and vegetables and more particularly to a coated porous membrane for use in controlling the flow of oxygen and carbon dioxide into and/or out of a produce container.
It is well-known that the eating qualities and/or appearance of respiring items such as fresh fruits, vegetables and flowers (hereinafter xe2x80x9cproducexe2x80x9d) can be preserved by controlling the atmosphere inside of the package that holds the produce. For example, U.S. Pat. No. 4,842,875, issued to H. Anderson on Jun. 27, 1989, describes a basic approach that involves controlling the flow of oxygen and carbon dioxide into and out of the container that holds the produce. The container, called a xe2x80x9ccontrolled atmosphere package,xe2x80x9d is comprised of a substantially gas-impermeable package having one or more panels made of a microporous plastic membrane having an oxygen permeability in the range of 77,500 to 465,000,000 cc/m2-atmosphere-day. By varying the permeability and/or size of the panel, various optimized oxygen and carbon dioxide levels inside the package can be maintained for extended periods of time thereby providing a method for retarding the maturation processes of various produce commodities.
U.S. Pat. No. 4,923,703, issued to M. Antoon on May 8, 1990, describes a microporous film for use as a panel in a controlled atmosphere package, comprised of a uniaxially oriented polyolefin film with an inert filler.
U.S. Pat. No. 4,910,032, issued to M. Antoon on Mar. 20, 1990, describes a controlled atmosphere package, having a first membrane comprised of a uniaxially or biaxially oriented polymer film that is permeable to oxygen and carbon dioxide, and a second membrane that is permeable to water but impermeable to oxygen and carbon dioxide.
U.S. Pat. No. 4,879,078, issued to M. Antoon on Nov. 7, 1989, and U.S. Pat. No. 4,923,650, issued to M. Antoon on May 8, 1990 describe methods for preparing microporous films that can be used in controlled atmosphere packaging for produce.
Additional patents that deal with this field include U.S. Pat. No. 4,939,030, issued to S. Tsuji et al. on Jul. 3, 1990, which discloses a three-layer film, including a vinyl acetate layer, for use in produce packaging; and U.S. Pat. No. 4,996,071, issued to L. Bell on Feb. 26, 1991, which discloses varying the surface area of the film to control the atmosphere inside of a produce package.
Additional patents dealing with this field but which are not considered to be material to the present invention include U.S. Pat. No. 3,625,876, issued to C. Fitko on Dec. 7, 1971; U.S. Pat. No. 4,769,262, issued to A. Ferrar et al. on Sep. 6, 1988; and U.S. Pat. No. 5,026,591, issued to R. Henn et al. on Jun. 25, 1991.
All of this prior art teaches that in order to establish and maintain different oxygen/carbon dioxide ratios inside of the package, either the permeability of the microporous membrane must be changed by altering the chemical formulation of the film used to make the membrane, or the size of the membrane panel must be changed.
The present invention comprises a coated porous membrane panel with a pin-hole aperture for use with a container that holds produce. The coated porous membrane panel with a small aperture is positioned in contact with, and over a hole in a substantially non-porous container for providing an area through which a limited volume of carbon dioxide and oxygen can flow at a rate (permeability) between the inside of the container and the outside ambient atmosphere which is within a range specific to the type and amount of produce in the package. The small aperture is positioned near the center of the hole in the container. The combination of the non-porous container and the coated porous membrane panel with a small aperture retards the maturation of the produce by allowing the concentration of oxygen and carbon dioxide inside the container to stabilize within a range of concentration reasonably optimal for the type of produce within the package. The coated porous membrane panel of the present invention may be comprised of a base substrate such as polypropylene or polyethylene, a nonwoven substrate made from these polymers, or paper (hereinafter xe2x80x9csubstratexe2x80x9d) which has a permeability rate to oxygen and carbon dioxide in a range of, for example, 77,500 to 465,000,000 cc/m2-atmosphere-day (5,000 to 30,000,000 cc/100 in2-atmosphere-day). This range is then reduced to a desired level suitable for the type and amount of produce to be packaged by applying to said substrate a coating of a porosity-reducing, film-forming substance, such as an acrylic-based polymer, which by coat weight (thickness) and/or pattern of application, decreases the permeability rate of the substrate to a desired range of permeance rates. The percent reduction in the oxygen and carbon dioxide permeability rate of the substrate can be varied 15 percent to almost 100 percent by varying the aforementioned characteristics of the coating applied to it. The small aperture allows the pressure inside of the container to approximate the pressure outside of the container preventing overpressurization and underpressurization without drying out the produce. Overpressurization or underpressurization of hermetically sealed produce packages occurs frequently as a result of produce respiration activities within packages or when packages are shipped over a mountainous range due to altitude changes.
The coated porous membrane panel of the present invention differs from the porous membrane panel described in the prior art (e.g., by Antoon in U.S. Pat. No. 4,879,078) in that the prior art varies the constituents of the membrane material to produce membranes of varying permeabilities, whereas in the present invention, a single substrate with a small aperture can be used to produce different membranes for different types and/or amounts of produce by simply varying the characteristics of a porosity-reducing coating material applied to the single substrate or by varying percent area on a single substrate where a coating is applied and in that the small aperture prevents overpressurization and underpressurization while maintaining relative humidity.